The present invention relates generally to the gas industry and in particular to a process for liquefaction of and nitrogen extraction from natural gas under pressure.
Liquefaction and nitrogen extraction processes of this type are well known to those skilled in the art, and have been used for numerous years. Cooling, lowering of pressure, and liquefaction and fractionation are carried out, in a known manner, in separate installations using appropriate means.
The process for liquefaction of natural gas is used in a known manner for the purpose of eliminating the nitrogen that is sometimes present in large quantity in the natural gas. This solution has the advantage of being economical from the standpoint of the energy consumption necessary for nitrogen extraction from and liquefaction of the natural gas.
In this case , the nitrogen produced contains large quantities of hydrocarbons (up to 50-60%), and therefore cannot be discharged directly into the atmosphere. This nitrogen, which is rich in hydrocarbons or fuel gas, can be used to supply gas turbines. This fuel gas can also be burned in the surplus gas flare.
In all cases, the burning of gas containing large proportions of nitrogen produces large quantities of nitrogen oxides which are significant atmospheric pollutants. Among the nitrogen oxides, certain ones are irritating and/or toxic gases which promote the appearance of respiratory diseases such as asthma. A public health problem arises from this, which does not allow installation, in urban zones, of nitrogen extraction units of known technology to be reasonably considered.
Moreover, during operation of the surplus gas flare, flames that can be over ten meters high are produced, thus presenting non-negligible safety risks.
Finally, the burning of fuel gas also produces large quantities of carbon dioxide which is responsible for a significant greenhouse effect, particularly because of its extensive absorption of light in the infrared domain.
The use of means for reducing the nitrogen oxides or else for absorbing the carbon dioxide contained in the exhaust vapors is known. However, these techniques are very expensive and would make the production of liquefied natural gas (LNG) unprofitable.
In this context, a first aim of the invention is to propose a process which enables one to eliminate pollution due to production of nitrogen oxides, and to limit the production of carbon dioxide.
More precisely, according to one of its first aspects, the invention relates to a process for liquefaction of and nitrogen extraction from a natural gas under pressure, containing methane, C2, d higher hydrocarbons and nitrogen, in order to obtain, on the one hand, a liquefied natural gas essentially free of nitrogen, and, on the other hand, gaseous nitrogen essentially free of hydrocarbons, which includes a first step (I) in which the natural gas is cooled, has its pressure reduced, is liquefied in order to provide a flow of pressure-reduced liquefied natural gas, and in which the pressure-reduced liquefied natural gas is separated, in a first fractionation column containing a number of stages, into a first top fraction that is relatively more volatile and that is nitrogen enriched, which is collected in the last stage of the first fractionation column, and a first bottom fraction that is relatively less volatile and that is nitrogen poor.
The process of the invention includes a second step (II) in which the first top fraction is cooled and liquefied in order to provide a first cooled and liquefied fraction, the first cooled and liquefied fraction is separated into a second top fraction that is relatively more volatile and very nitrogen enriched, and into a second bottom fraction that is relatively less volatile and relatively less nitrogen enriched, the second bottom fraction is introduced into the last stage of said first fractionation column, the second top fraction is heated in order to provide gaseous nitrogen, and the first bottom fraction is cooled in order to provide liquefied natural gas that is essentially free of nitrogen.
Thus, the discharges mainly consist of nitrogen, and are essentially free of hydrocarbons.
One of the merits of the invention is the discovery of an economical solution, by comparison to known means of reducing nitrogen oxides and eliminating carbon dioxide produced by the combustion of nitrogen-rich gases containing hydrocarbons. The invention shows that it is possible (a) to effectively separate the nitrogen from the natural gas and (b) to produce, on the one hand, nitrogen, essentially free of hydrocarbons, that can possibly be discharged without treatment into the atmosphere, and on the other hand, liquefied natural gas, in a single operation. The invention therefore enables one to avoid burning in general, and in particular, burning of nitrogen-rich gases issuing from natural gas nitrogen extraction, while limiting discharges of atmospheric pollutants.
Another merit of the invention is that the steps of liquefaction of and nitrogen extraction from the natural gas are combined in a single installation. This combination has the surprising advantage of reducing the costs of production of the liquefied natural gas, while making it possible to limit the environmental impact of the gaseous discharges. Moreover, the process of the invention uses a limited number of steps, consequently allowing simplified production because of the limited number of elements to be assembled.
The process of the invention can furthermore include a third step (III) in which the first bottom fraction, before being cooled in order to obtain liquefied natural gas essentially free of nitrogen, is cooled, has its pressure reduced, and is introduced into a first relatively warmer stage of a second fractionation column, and the second top fraction, before being heated in order to provide gaseous nitrogen, is cooled, has its pressure reduced, is liquefied, and is introduced into a second relatively colder stage of said second fractionation column.
The addition of this third step makes it possible to reduce the compressive power necessary for cooling fluids circulating in an installation according to the process.
The invention can also provide for the cooling and heating operations to be carried out in a number of heat exchangers, and for each of them to be supplied with one to three separate issuing fluids coming respectively from one to three separate cooling cycles.
According to a preferred embodiment of the invention, the cooling fluid(s) is(are) mixtures.
According to a preferred embodiment of the invention, a number of compressors are used for compressing the cooling fluid(s), and preferably, the compressors operate using electric motors.
The electricity can be produced by hydraulic, solar or nuclear power stations which do not produce any greenhouse gas.
According to a second of its aspects, the invention relates to a liquefied natural gas which is essentially free of nitrogen, and to nitrogen essentially free of hydrocarbons, obtained by a process according to the invention.
The (LNG) obtained by the process of the invention will usually and preferably contain less than 1 mol % of nitrogen, and the gaseous nitrogen obtained by the process of the invention will usually and preferably contain less that 1 mol % of hydrocarbons.
According to a third of its aspects, the invention relates to an installation for liquefaction of and extracting nitrogen from natural gas under pressure containing methane, C2, and higher hydrocarbons and nitrogen, for obtaining, on the one hand, a liquefied natural gas essentially free of nitrogen, and on the other hand, gaseous nitrogen essentially free of hydrocarbons, which includes means for carrying out a first step (I) in which the natural gas is cooled, has its pressure reduced, and is liquefied in order to provide a flow of pressure-reduced liquefied natural gas, the pressure-reduced liquefied natural gas is separated, in a first fractionation column containing a number of stages, into a first top fraction that is relatively more volatile and that is nitrogen enriched, which is collected in the last stage of the first fractionation column, and a first bottom fraction that is relatively less volatile and that is nitrogen poor, including means for carrying out a second step (II) in which the first top fraction is cooled and liquefied in order to provide a first cooled and liquefied fraction, the first cooled and liquefied fraction is separated into a second top fraction that is relatively more volatile and very nitrogen enriched, and into a second bottom fraction that is relatively less volatile and relatively less nitrogen enriched, the second bottom fraction is introduced into the last stage of the first fractionation column, the second top fraction is heated in order to provide gaseous nitrogen, and the first bottom fraction is cooled in order to provide liquefied natural gas that is essentially free of nitrogen.
This installation has the advantage of using a limited number of elements for implementation of the process of the invention.
The installation of the invention can moreover include means for carrying out a third step (III) in which the first bottom fraction, before being cooled in order to obtain liquefied natural gas essentially free of nitrogen, is cooled, has its pressure reduced, and is introduced into a first relatively warmer stage of a second fractionation column, and the second top fraction, before being heated in order to provide gaseous nitrogen, is cooled, has its pressure reduced, is liquefied, and is introduced into a second relatively colder stage of said second fractionation column.
The installation of the invention can moreover provide for the cooling and heating operations to be carried out in heat exchangers, and for each of them to be supplied with one to three separate cooling fluids issuing respectively from one to three separate cooling cycles.